1. The Field of the Invention
Embodiments of the present invention relate generally to electron emitters. More particularly, embodiments of the present invention relate to thermionic emission of electrons for x-ray generation.
2. The Relevant Technology
The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
An x-ray tube typically includes a vacuum enclosure that contains a cathode assembly and an anode assembly. The vacuum enclosure may be composed of metal such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. At least a portion of the outer housing may be covered with a shielding layer (composed of, for example, lead or a similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating it to an external heat exchanger via a pump and fluid conduits. The cathode assembly generally consists of a metallic cathode head assembly and a source of electrons highly energized for generating x-rays. The anode assembly, which is generally manufactured from a refractory metal such as tungsten, includes a target surface that is oriented to receive electrons emitted by the cathode assembly.
During operation of the x-ray tube, the cathode is charged with a heating current that causes electrons to “boil” off the electron source by the process of thermionic emission. An electric potential on the order of about 4 kV to over about 200 kV is applied between the cathode and the anode in order to accelerate electrons boiled off the electron source toward the target surface of the anode assembly. X-rays are generated when the highly accelerated electrons strike the target.
Most of the electrons that strike the anode dissipate their energy in the form of heat. Some electrons, however, interact with the atoms that make up the target and generate x-rays. The wavelength of the x-rays produced depends in large part on the type of material used to form the anode surface. X-rays are generally produced on the anode surface through two separate phenomena. In the first, the electrons that strike the cathode carry sufficient energy to “excite” or eject electrons from the inner orbitals of the atoms that make up the target. When these excited electrons return to their ground state, they give up the excitation energy in the form of x-rays with a characteristic wavelength. In the second process, some of the electrons from the cathode interact with the atoms of the target element such that the electrons are decelerated around them. These decelerating interactions are converted into x-rays by conservation of momentum through a process called bremstrahlung. Some of the x-rays that are produced by these processes ultimately exit the x-ray tube through a window of the x-ray tube, and interact with a patient, a material sample, or another object.
In order to produce high-quality x-ray images it is generally desirable to maximize both x-ray flux (i.e., the number of x-ray photons emitted per unit time) and x-ray beam focusing. An intense electron beam is useful for collecting high-contrast images in as short a period of time as possible, while the ability to distinguish between different structures in an x-ray image (e.g., a cancerous mass versus surrounding healthy tissue) is limited by x-ray focusing.
X-ray flux can be increased by increasing the number of electrons emitted by the emitter that impinge on the target anode. The number of electrons emitted by the emitter is a function of the amount of electrical current passing through the emitter and the temperature of the emitter. In general, raising the current increases the temperature of the emitter, which increases the number of electrons emitted by the emitter. In turn, greater x-ray flux is produced when greater numbers of electrons strike the target surface.
While image contrast depends on electron flux, image quality (i.e., the ability to distinguish between different structures in an x-ray image) is a function of the focal pattern, or focal spot, created by the emitted beam of electrons on the target surface of the target anode. In general, a smaller focal spot produces a more highly focused or collimated beam of x-rays, which in turn produces better quality x-ray images. This phenomenon can readily be analogized to the shadows produced by a visual light source. For example, the shadows cast by a sharp light source (e.g., a point source such as a laser) are themselves sharp, while the shadows cast by a poorly defined light source (e.g., fluorescent office lights) are themselves poorly defined and diffuse. The same is true of the shadows cast by the x-rays that are transmitted and absorbed as x-rays pass through a subject.
Nevertheless, the desire to maximize electron flux and the desire to maximize electron beam focusing are often at odds with one another. For example, raising the temperature of the emitter to increase electron beam flux can cause the shape of the emitter to change, which can adversely affect electron beam focusing. In extreme cases, increasing the amount of current passing through the emitter can damage the emitter leading to failure of the x-ray device.
Another important consideration in the design of x-ray devices is the physical limits of the anode. As mentioned above, the majority of the electrons that impinge on the target anode dissipate their energy in the form of heat rather than generating x-rays. In order to maximize x-ray flux, it is generally necessary to apply the maximum possible power to the emitter, which heats the anode to its physical limits. A lack of homogeneity in anode heating produced by the electron beam will limit the amount of power that can be applied to the x-ray device and limit the x-ray flux that can be obtained. Anode overheating and electron beam inhomogeneity are usually alleviated—but not eliminated—by rotating the anode at high speed.